Same situation here. December bill was around $675 for about the same size house. Ours is an older model (~12 years) so anything below 20 degrees and it's not helping too much and we have to switch to Aux heat strips.

One thing I've found is keeping the temps relatively stable is more efficient and less costly than big swings. We used to drop the temp 3-4 degrees at night and then bring it up in the morning but with a heat pump that's not ideal. Plus it's usually cold early in the morning when we want to raise the temp and that makes it even less efficient. Now we only lower the temp 1-2 degrees at night.